In my recollection, the rods with castellated nuts had the washer there so that the washer could be dressed with a file to get the castellations to match a split-pin hole.
Late large journal BSA rods did vary quite a bit in the amount of clearance left for a socket spanner to tighten them (I bought numbers of these rods back in the '60s straight from BSA). On the later rods the spanner clearance was very small and I have a socket spanner turned down to "quite thin" to undo these little so-and-sos. 28 lbs ft and red Loctite always kept them happy. Two types of nut were fitted, firstly ones with a crimped on interference washer then the type with 2 slits in them, with the slits closed up to interfere with the bolt threads. In the 70s and 80s there were some truly awful bolts around, I once bought some where the threads were all torn - clearly not rolled threads, and cut with a blunt die. These went into the circular filing cabinet. I have never seen washers fitted under self-locking nuts.
In the early 2000s I went over to the ARP bolts sold by SRM and they really are fine engineering. Because the nuts are quite tall I had problems with their tips catching the inside of the crankcase but some filing saw to that. I think if you want to stick to 5/16" bolts then these are the best choice. ARP's reputation rest on their bolts!
My current motor has the billet rods made by R&R in the USA and sold by Wassell (I never thought Wassell would get into that territory!). These are very good rods, CNC with corrugated joint faces and 3/8" x 24 ANF bolts (no nuts) made by ARP done up to something like 50 ft lbs - twice the pressure of any 5/16" bolt.
